SAN JOSE, Calif. -- A woman was reunited with her two children after a nightmarish 18 months during which they were held for ransom in Mexico, with the captors demanding more and more money and threatening to kill her if she tried to reclaim them or told police, according to a federal indictment.

The U.S. Department of Justice made the announcement Monday that the children -- whose ages were not specified -- have been back with their mother since Friday, after a law enforcement effort that crossed state and international lines.

According to the indictment, the unnamed mother, who is a Mexican national, gave birth to her children in the United States and was living in Washington state until 2008, when she returned to Mexico with her children. She stayed there a few years but illegally returned to the United States in search of work, leaving her children with their grandmother in Mexico until she could make arrangements for them to join her.

The indictment states that's when she came into contact with Jesus Salinas and Patricia Delatorre of Mt. Vernon, Wash. The pair -- also Mexican nationals -- agreed to bring the children to their mother for a fee.

After receiving $1,300, the children were placed in the care of Maria Guadalupe Valenzuela Castaneda in Juarez, Mexico, in June 2011. But the trio said they wouldn't bring the children north until more payments were made. She eventually sent them about $5,000.

"For months, the mother sent payments to the conspirators, but the children were never returned," reads a DOJ news release. "The conspirators also threatened to kill the mother if she contacted law enforcement or if she went to Juarez in search of the children."

Last December, the FBI, working with Mexican authorities, located the children and took them from Castaneda. They were placed in the care of Mexican social services until Friday.

"The FBI worked closely with our law enforcement partners in California, Washington, Texas and Mexico to safely recover these children from their kidnappers and reunite them with their mother," stated FBI officials. "We will continue to actively pursue and bring to justice those individuals who kidnap children and extort family members for money. These crimes take a terrible toll on the victims and we will hold the perpetrators responsible."

The indictment charges the defendants with one count of kidnapping minor victims in interstate and foreign commerce, and two counts of unlawful seizure and detention of a U.S. citizen. They face 20 years to life in prison and fines of up to $500,000.

Salinas and Delatorre were arrested in Washington in January and remain in custody. An arrest warrant was issued for Castaneda, and authorities ask anyone with knowledge of her whereabouts to contact the FBI at 408-369-8000.